1. Field of the Invention
The present invention relates to an ignition circuit for vehicle occupant protecting system which is used for actuating a vehicle occupant protecting system such as a gas bag system.
2. Description of the Prior Art
Ignition circuits for vehicle occupant protecting systems are known in the art in which in response to the output signal of a collision detecting sensor for detecting a collision, a heating element such as a filament is ignited through a single switch means to actuate the detonator of a gas bag.
A disadvantage of this type of ignition circuit is that if the final stage switch for igniting a filament is broken down, there is a substantial danger of causing the filament to be ignited and thus causing the gas bag to operate spontaneously. Particularly, where the switch means comprises contactless switch means which consists of a semiconductor element which is connected in series with a filament, there is involved a very great danger in consideration of the fact that most of such semiconductor elements tned to conduct current when they are broken.
In the other hand, where the switch means consists of the contacts of a mechanical switch such as an acceleration/deceleration sensor switch, there is also similar danger of the gas bag operating erroneously when the switch means is exposed to harmful environments, such as sea water or atmosphere containing salt, which causes the contacts to become faulty and remain in the conductive state.